How to add image texture to an existing material?
You can use object texture coordinates to do that:

In the node above, object texture coordinates are driven by the position of an empty.
Then the mapping node shifts things by 0.5 to center the decal image around the empty (on left for the cube the material has no mapping).
Now you can adjust the empty position as you need, using the snap tool, for instance (Z must be up).

The mix color node (before the shader) can be replaced by a mix shader node (after the shader) as you wish.
You can use the setting several times, if needed:

The image size and rotation depend on the empty size and rotation.